ATP-binding cassette (ABC) Transporters represent the molecular architecture most commonly employed by cells to drive the active transport of solutes across membranes using adenosine triphosphate (ATP) as an energy source. ABC Transporters occur in all living organisms and comprise one of the largest gene family in fully sequenced microbial genomes. At least one bacterial ABC Transporter has been shown to be essential for the viability of E. coli, while others have been shown to function as bacterial pathogenicity factors. The aberrant function of human ABC Transporters plays a central role in a variety of diseases including cystic fibrosis, adrenoleukodystrophy, and multidrug resistance in advanced tumors and leukemias. Previous research from my lab has helped to define the mechanism by which the structurally-stereotyped ABC motor domains in ABC Transporters employ the binding and hydrolysis of ATP to perform mechanical work. Using combined enzymological and crystallographic studies, we showed that ATP binding to conserved sequence motifs drives the formation an "ATP-sandwich dimer" by the ABC motor domains. This ATP-induced dimerization is believed to represent the power-stroke of the pumps because the physical movements of the ABC's during dimer formation is hypothesized to drive the essential conformational rearrangements in the transmembrane domains that mediate solute transport. We propose to extend our earlier studies of the mechanochemical reaction cycle of ABC Transporter motor domains to characterize the structural details of these allosteric conformational changes in the transmembrane domains that drive solute transport in one specific class of ABC efflux pumps. Our proposed studies involve a comgination of genetic, enzymological, and crystallographic approached to this fundamental biophysical problem. Successful completion of this work would provide deeper structural and mechanistic understanding that would facilitate studies on the molecular pharmacology of the disease-causing proteins in the ABC Transporter superfamily.